Tin plate treatment and product



Patented Apr. 11, 1967 3,313,714 TIN PLATE TREATMENT AND PRODUCT Jack E. Joyce, Chesterton, and Robert Lozano, Griffith, Ind., assignors to Inland Steel Company, Chicago, 111., a corporation of Delaware No Drawing. Filed Nov. 16, 1964, Ser. No. 411,617 7 Claims. (Cl. 20432) The present invention relates generally to a method of treating tin plate to improve the surface properties of the tin plate and to the treated tin plate produced. More particularly, the present invention relates to an improved electrolytic tin plate having improved stain resistant properties and to a method of treating electrolytic tin plate to improve its stain resistant properties.

Much of the tin plate used for making cans and other containers, particularly containers for foods and beverages, is produced commercially by the electroplating of steel strips with a thin coating of metallic tin. The electrolytic tin plate used for making containers for foods and beverages should have the necessary physical and chemical properties to withstand both external and internal attack, including good resistance to corrosion, exhibit good adherence for protective and decorative coatings, and have good resistance to discoloration or staining by foods and beverages which are packed therein, such as those foods which are acidic andparticularly those having sulfurcontaining constituents.

In producing electrolytic tin plate commercially, a continuous strip of steel, such as blackplate, is progressively moved through a long electroplating line. As the strip moves through the line, usually at speeds of about one thousand feet per minute, it is cleaned, pickled, scrubbed, and then electroplated with a coating of tin. After the strip has been plated, usually on both surfaces, it is washed to remove any residual electrolyte and the tin coating is flow brightened to convert the tin from a condition in which it presents a matte surface to a condition in which it presents a bright mirror-like surface. The flow brightening is effected by melting and solidifying the tin coating. The flow brightened tin plate strip is then usually treated chemically and wound into large coils for storage or shipping. Subsequently, the coil is trimmed, cut and thereafter processed to form the containers. An important factor in determining the properties of the electrolytic tin plate is the chemical treatment applied to the tin plate immediately following the flow brightening ste i great many chemical and electrochemical filming treatments have been developed heretofore which have as their objective the removal of contaminants and retardation of the growth of objectionable oxide films, thereby rendering the tin plate surface resistant to oxidation and improving the properties thereof. The more successful of these treatments all use a soluble chromium compound in at least one of the treating solutions which may or may not include an electrolytic step. None of the prior art methods of treatin tin plate, however, have been able to impart entirely satisfactory stain resistant properties to electrolytically produce tin plate having considerable surface oxide formation, particularly tin plate produced on an alkaline electrolytic tin line after the tin surface has been fused to provide the desired smooth bright surface.

It is therefore an object of the present invention to provide a method of improving the stain resistant properties of electrolytic tin plate.

It is also an object of the present invention to provide a method of treating tin plate to improve the stain resistant characteristics thereof while also imparting good lacquer adherent properties.

It is a further object of the present invention to provide an improved electrolytic tin plate product characterized by improved stain resistant properties.

Other objects of the present invention will be apparent to those skilled in the art from the accompanying detailed description and claims to follow.

It has been discovered that electrolytic tiri plate having very substantially improved stain resistant properties can be produced by subjecting a tin plate surface after fusion thereof to treatment by a combination of steps comprising:

( l) Cathodic cleaning in an alkaline electrolyte (2) Water rinse (3) Dilute acid dip to bring the between about pH 2 and 6 (4) Cathodic chromate treatment in an electrolyte having a pH of between about 2 and 6 at a high current density (5) Dilute acid treatment in a non-toxic acid solution having a pH of between about 2 and 6.

surface of the plate to The method of treating tin plate comprising the foregoing novel combination of steps is particularly suited for use on tin plate which has a relatively large amount of oxide on the surface, such as produced by a gas fusion furnace or by electrolytic action, and which has not heretofore had satisfactory stain resistant properties. It should be understood, however, that the process of the present invention is not restricted to treating tin plate produced on an alkaline or an acidic electrolytic tin plate line by any particular method.

In treating tin plate by the process of the present invention, it is necessary to first clean the tin plate by removing surface contaminants and oxides produced during or after tin plating and fusion. The surface contaminants and oxides on the tin plate are substantially removed in the first step of the instant process by cathodically treating the tin plate in a relatively dilute alkaline electrolyte, such as a sodium carbonate solution. Thus, for example, a sodium carbonate solution having a concentration between about 1% and 10% by weight, and preferably between 3 and 5% by Weight, when correlated with temperature, time of treatment, and current density to avoid reacting with the tin, has produced good oxide removal and cleaning results. However, a solution of any equivalent alkali metal compound can be used in equivalent concentrations and under equivalent conditions. Other compounds or mixtures thereof which can be used in place of sodium carbonate are sodium hydroxide, tri-sodium phosphate, potassium carbonate, potassium hydroxide, sodium bicarbonate, mono-sodium hydrogen phosphate, disodium hydrogen phosphate, and potassium bicarbonate. The current density in the electrolyte can be maintained between about 20 and 60 coulombs/ft while the temperature of the electrolyte is between about to 200 F., and preferably at about ISO- F.

After being thus briefly cathodically cleaned in the alkaline electrolyte, the tin plate is washed with water, and is immersed in an acidic conditioning bath prior to being subjected to a cathodic passivating treatment. The acidic conditioning treatment to which the cleaned tin plate is subjected in the instant process comprises immersing the tin plate in a dilute acidic solution having a pH of between about pH 2 and 6, and preferably between pH 3.5 and 5.0, which corresponds substantially to the pH of the subsequently applied cathodic passivating treatment step. The acidic conditioning bath can comprise an aqueous solution of a water soluble organic or inorganic acid. Among the preferred organic acids which can be used are benzoic, propionic, butyric, valeric, carbonic, acetic acid, formic acid, citric acid, oxalic and tartaric acid with acetic acid being preferred. Inorganic acids which can be used include hydrochloric acid and hydrofluoric acid. The acid conditioning solution is preferably prepared by using demineralized or soft water to avoid extraneous contamination. With the aqueous conditioning solution acidified with an organic acid, such as acetic acid, to a pH of preferably between pH 3.5 and 5, the conditioning solution is maintained between about 100 to 200 F. and preferably at about ISO-170 F., the latter temperature being sufiicient to heat the tin plate to substantially the temperature of the electrolyte in the subsequent passivating treatment. The tin plate is allowed to remain in the acid bath for a period of only about 1-3 seconds, after which it is withdrawn and subjected directly to a cathodic passivating electrolyte treatment.

The cathodic passivating treatment which is applied to the conditioned tin plate in the instant process comprises making the tin plate the cathode in a heated aqueous electrolyte containing hexavalent chromium ions with the electrolyte having a pH of between about pHZ and 6, and preferably between pH 3.5 and 5, while passing an electric current through the electrolyte at a high current density of between about 60 and 80 coulombs per square foot. Whereas sodium dichromate is the preferred hexavalent chromium compound for the cathodic passivating treatment, any other hexavalent chromium compound or compound which is converted to a hexavalent chromium compound can be used, including sodium chromate, potassium chromate, potassium dichromate, and chromic acid. The hexavalent chromium ion concentration in the electrolyte can range between a concentration of hexavalent chromium ion equivalent to about 1% to about 10% by weight of sodium dichromate, and preferably equivalent to about 23% by weight sodium dichromate, so that the pH of the electrolyte does not depart appreciably from the preferred range of about pH 3.5 and 5 and remains Within the broad range of pH 2 to 6. The temperature of the hexavalent chromium containing electrolyte bath can be maintained between about 100 F. and 200 F. but preferably is held between about 160 F. and 180 F. It should be understood, however, that the limits specified for each of the operating conditions described herein are interdependent and also depend on :the manner and period of time during which the tin plate is allowed to remain in contact with the electrolyte. Thus, in the foregoing discussion the ranges specified are applicable generally to a process in which an endless strip of tin plate is passed continuously through an electrolyte bath at a rate of about 500 to 1200 feet per minute.

Since the pH of the acid conditioning bath is substantially the same as the pH of the passivating electrolyte there is no water rinse required between the conditioning and passivating steps. And, while some carry-over of the acid solution to the acidic cathodic passivating electrolyte takes place, the presence of an acid, such as the preferred acetic acid, in the hexavalent chromium passivating electrolyte and under some circumstances further improves the surface properties of the tin plate.

Following the electrolytic hexavalent chromate step the tin plate is subjected to a critical final acid treatment wherein the thin strip is passed through a dilute aqueous solution of a non-toxic acid which does not leave an objectionable deposit or residue on the tin surface. The acids which can be used also should be, as applied to the surface of the tin plate, non-oxidizing and include benzoic, propionic, butyric, citric, valeric, carbonic, formic, and acetic acid, with acetic acid being preferred. The pH of the final acid bath should range between about pH 2 and 6 with a preferred pH of between pH 3.5 and 5.0. The temperature of the acid bath can be between 180 F. and 212 F. with a preferred temperature of about 190 F. The period of contact between the tin plated strip and the acid bath can be from .2 seconds to 60 seconds with a preferred contact time of between 1 and 3 seconds. It is also important for best results to limit the concentration of the hexavalent chromate in the acid bath to a maximum of 1.0 gram per liter. If desired, the strip emerging from the electrolytic chromate treatment can be washed with water as it leaves the chromate bath. Thereafter the strip is dried and oiled in the conventional manner and wound into suitable coils for shipment.

In order to further illustrate the present invention the following specific example is set forth without, however, limiting the invention to the particular conditions or reagents employed:

Tin plate produced electrolytically on an alkaline tin plating line in endless strip form and after conventional flow-brightening in a gas fusion furnace is passed continuously at a line speed rate of about 700 ft. per minute through an alkaline electrolyte cleaning solution comprising about 4 percent by weight of sodium carbonate. The strip is made the cathode while immersed in the alkaline electrolyte and an electric current having a current density of about 25 coulombs per ft. is passed through the electrolyte and the strip while maintaining the electrolyte at a temperature of about l70 F. The strip is moved from the sodium carbonate electrolyte through a water Wash and then passed between air jets which remove all the wash and then passed between air jets which remove all the wash water from the surface of the strip. The strip next enters a non-electrolytic acid conditioning bath comprising an aqueous acetic acid solution having a pH between 3.5 and 6 comprising about .01 percent by volume glacial acetic acid. The acid solution is heated to a temperature of about ISO- F. Thereafter the strip is passed directly into a heated aqueous sodium dichromate electrolyte containing about 30-40 grams sodium dichromate per liter and having a pH of about pH 4.5. The electrolyte is maintained at a temperature of about 160-180 F. Upon entering the dichromate electrolyte, the strip is made the cathode and an electric current having a density about 70 coulombs per ft. is passed through the electrolyte and the strip. The strip is rinsed with water as it leaves the dichromate electrolyte and passed through a dilute aqueous acetic acid treating bath having a pH between about 4.0 and 4.5 and maintained at a temperature of about F. The strip remains immersed in the acid treating bath for a period of between 1 and 3 seconds and preferably for about 2 seconds. The acid treated, dried tin plated strip is then preferably oiled or otherwise coated with a protective surface film prior to coiling or the strip may be trimmed and cut without coiling, if desired.

The tin plate produced in the foregoing manner is highly resistant to discoloration due to sulfur containing foods and beverages. Cans made in a conventional manner from the tin plate produced in the foregoing manner were packed with green pea soup and vegetarian soups, respectively, and stored at approximately 120 F. After eight weeks storage at 120 F. which is considered equivalent to two years storage at room temperature, the cans packed with green pea soup showed the least sulfide staining and the least detinning with vegetarian vegetable soup of any tin plate heretofore tested.

It will be understood by those skilled in the art that the process of the present invention, as illustrated in the foregoing specific example, can be carried out with a combination of conventional equipment presently employed for alkaline cathodic cleaning, water washing, chromic acid dipping, and cathodic dichromate passivating treatments.

It should also be understood that various modifications can be made in the individual steps of the process without departing from the present invention comprising the novel combination of steps herein described in order to adapt the invention for use with a given electroplating line or hot dip galvanizing line, whereby the herein described improved surface characteristics are imparted, particularly improved lacquer adherent properties, and also resistance to discoloration.

We claim:

1. A method of treating tin plate to provide improved 7 5 stain resistant surface characteristics which comprises, im-

mersing said tin plate in a hot aqueous alkaline cleaning electrolyte while passing an electric current through said electrolyte and said tin plate to remove substantially all surface oxides and other contamination thereon without removing tin, washing said tin plate with water to rinse away residual alkaline cleaning solution, applying to said tin plate a dilute acid solution having a pH between about pH 2 and 6, passivating the acid conditioned tin plate surface by making said tin plate cathodic while immersing in a hot aqueous hexavalent chromium solution having a pH between about pH 2 and 6 and passing an electric current at a high current density of at least 60 coulombs per ft. through said tin plate and said chromium containing electrolyte, and thereafter contacting said tin plate with a final acidic treating solution comprising a dilute aqueous non-toxic non-oxidizing acid solution having a pH between about pH 2 and 6; whereby a tin plate with substantially improved stain resistant surface characteristics is provided.

2. A method as in claim 1, wherein said acid solution, said hexavalent chromium solution, and said final acidic treating solution each have a pH between about pH 3.5 and 5.0.

3. A method as in claim 1, wherein said final acidic treating solution is a dilute aqueous solution of acetic acid which has a pH between about 3.5 and 5.

4. A method of treating tin plate to provide improved stain resistant surface characteristics which comprises, immersing tin plate in a hot aqueous alkaline cleaning electrolyte while passing an electric current through said electrolyte and said tin plate to remove substantially all surface oxides and other contamination thereon without removing tin, washing said tin plate with water to rinse away residual alkaline cleaning electrolyte remaining on the tin plate, applying to said tin plate a dilute aqueous acid solution having a pH between about pH 2 and 6, passivating said tin plate by making said tin plate cathodic while immersing in a hot aqueous heXavalent chromium containing electrolyte having a pH between about pH 2 and 6 and passing an electric current at a high current density of between and coulombs per ft. through said tin plate and said chromium containing electrolyte, and contacting said tin plate with a final acidic treating solution comprising a dilute aqueous non-toxic non-oxidizing acid solution having a pH between about pH 2 and 6; whereby a tin plate with substantially improved stain resistant surface characteristics is provided.

5. A method as in claim 4, wherein said acid solution, said hexavalent chromium solution, and said final acidic treating solution each have a pH between about pH 3.5 and 5.0 and a temperature of between about F. and 200 F.

6. A method as in claim 4, wherein said final acidic treating solution is a dilute aqueous solution of acetic acid which has a pH between about 3.5 and 5.

7. A tin plate product having improved acidic and sulfur stain resistant surface characteristics when made by the process of claim 4.

References Cited by the Examiner UNITED STATES PATENTS 2,215,165 9/1940 Summer 204141 2,312,076 2/1943 Cook et al 204141 2,314,818 3/1943 Cook et al. 204141 2,327,127 8/1943 Rath 204-441 2,450,509 10/1948 Glock 204-56 2,503,217 4/1950 Prust 204-141 2,606,866 8/1952 Neish 204-37 2,775,535 12/1956 Poole 1486.2 2,931,759 4/1960 Hill 20435 2,974,091 3/1961 Neish 20437 3,138,548 6/1964 Ham et a1 20432 HOWARD S. WILLIAMS, Primary Examiner. R. K. MIHALEK, Assistant Examiner. 

1. A METHOD OF TREATING TIM PLATE TO PROVIDE IMPROVED STAIN RESISTANT SURFACE CHARACTERISTICS WHICH COMPRISES, IMMERSING SAID TIN PLATE IN A HOT AQUEOUS ALKALINE CLEANING ELECTROLYTE WHILE PASSING AN ELECTRIC CURRENT THROUGH SAID ELECTROLYTE AND SAID TIM PLATE TO REMOVE SUBSTANTIALLY ALL SURFACE OXIDES AND OTHER CONTAMINATION THEREON WITHOUT REMOVING TIN, WASHING SAID TIN PLATE WITH WATER TO RINSE AWAY RESIDUAL ALKALINE CLEANING SOLUTION, APPLYING TO SAID TIN PLATE A DILUTE ACID SOLUTIO HAVING A PH BETWEEN ABOUT PH2 AND 6, PASSIVATING THE ACID CONDITINED TIN PLATE SURFACE BY MAKING SAID TIN PLATE CATHODIC WHILE IMMERSING IN A HOT AQUEOUS HEXAVALENT CHROMIUM SOLUTION HAVING A PH BETWEEN ABOUT PH 2 AND 6 AND PASSING AN ELECTRIC CURRENT AT A HIGH CURRENT DENSITY OF AT LEAST 60 COULOMBS PER FT2 THROUGH SAID TIM PLATE AND SAID CHROMIUM CONTAINIG ELECTROLYTE, AND THEREAFTER CONTACTING SAID TIN PLATE WITH A FINAL ACIDIC TREATING SOLUTION COMPRISING A DILUTE AQUEOUS NON-TOXIC NON-OXIDIZING ACID SOLUTIO HAVING A PH BETWEEN ABOUT PH 2 AND 6; WHEREBY A TIN PLATE WITH SUBSTANTIALLY IMPROVED STAIN RESISTANT SURFACE CHARACTERISTICS IS PROVIDED. 